It is known that the focusing of a beam of monopolar charged particles can be controlled by means of the action of a magnetic flux density field. Beams of this type, especially electron beams, are used, for example, to vaporize substances in vacuum coating systems. It is important in this case to be able to change, in a controlled manner, the cross-sectional area of the beam striking the material to be vaporized as well as its shape or extent and position; that is, it is important to be able not only to focus the beam in a controlled manner but also to deflect it in a controlled manner. The goal of controlled focusing, that is, of controlling the shape or size of the cross-sectional area striking the material to be vaporized, is, for example, to bring about the uniform removal of the material to be vaporized and/or to make allowances for the many different thermal characteristics of the various materials being vaporized.
When focusing is controlled by means of magnetic flux density fields, advantage is taken of the fact that moving charged particles such as electrons or ions are subjected in such a field to a force which is proportional to the charge of the particle being considered but also proportional to the cross-product of the particle velocity v times the magnetic flux density B.
A process of this type is known from U.S. Pat. No. 4,064,352.
According to that document, pole shoes extend along both sides of the electron beam, between which a largely homogeneous magnetic field is generated by means of a magnet.
The field is nonhomogeneous in the narrow air gaps between the inward-projecting magnetic cores.
In addition, W. German Patent No. 2,719,725 discloses an electromagnetic beam focusing technique for an essentially flat beam in which, based largely on a quadrupole system, an encompassing rectangular frame core is provided in a plane perpendicular to the direction of beam propagation; winding arrays are provided on the shanks of this core. The fields controlling the focusing, which are antiparallel, extend for a certain distance in a way which corresponds to the extension of the flat beam. No provision is made for shifting the beam, as in the case of an adjustable beam deflecting system.
The present invention has the goal of creating a process and a system of the type indicated above by means of which the focusing can be adjusted in one direction within certain limits, independently of the position of the beam in the direction transverse thereto or in which the adjusted focusing remains unaffected when the beam is shifted in the transverse direction indicated.
In connection with the present invention, additional windings are of particular interest.
It can be seen in particular from this that the components of the field vectors directed toward the opposing magnetic cores exert a deflecting effect on the electron beam in the direction transverse thereto and that, even temporarily without consideration of the electromagnets, the transverse components of the magnetic field vectors in question, of the fields between the opposing pairs of cores, act in opposition to each other.
If now, by means of the system of electromagnets, the field in the air gap between one pair of cores is intensified and the one in the air gap between the other pair is weakened to the same extent, the deflecting field component in the direction from one core to the other remains constant precisely on one coordinate, such as on a plane of symmetry between the two air gaps of the pairs of cores, and then, in spite of the change in the field intensities, the electron beam remains unaffected by the deflecting field components in the direction from core to core, that is, by the pairs of cores, when, which is true only in the ideal case, the transverse extent of the electron beam is negligible. Otherwise, the deflecting forces acting on the electrons are different on the left from those on the right, and thus the point of greatest electron density shifts across the beam, which is equivalent to a change in beam deflection in the transverse direction and to a change in the way it is focused in this direction.
This effect is ignored in the document mentioned. It is assumed that, when the field change mentioned occurs, the change in the components in the direction of the pairs of cores does not affect the beam. Advantage is taken of the change in the field vector components in the transverse direction. If, as explained by way of example, the field is intensified in the air gap of one of the pairs of cores but weakened in the other air gap of the other pair, the transverse vector components in the first air gap will now be dominant.
This results in vector components in the transverse direction on both sides of the electron beam; these vector components are by definition parallel to each other and have opposing directions. This is how the focusing of the electron beam is influenced in the core pair direction.
By means of the system just described, it is impossible to adjust the focusing of the beam without simultaneously changing how it is deflected. Nor is it possible to change the position of the beam transversely to the focusing effect without readjusting the focusing control fields.